Bubbles and other voids trapped within a cured composite laminate comprise the “porosity” of the laminate. An optimally cured composite laminate may have little or no porosity (sometimes described as 0% porosity). The percentage of porosity is defined as the ratio of the part's volume comprised of voids to the volume comprised of solids. In some applications, porosity may weaken a part and render it useless.
In industries such as the aerospace industry in which composite structures are utilized, it may be necessary to determine the porosity of such structures. Accurate measurement of porosity in composite structures may be important in both composite manufacturing and repair. Porosity measurements applied to composite structures may be carried out using ultrasonic attenuation measurements. Ultrasonic attenuation measurement of composite structures is based on the attenuation of ultrasonic energy which occurs by one of two mechanisms when ultrasonic energy impinges against the composite structure. According to the first mechanism, the ultrasonic energy is scattered at interfaces between adjacent structures in a process known as interfacial losses. According to the second mechanism, the ultrasonic energy is attenuated as it propagates through the thickness of the structure in a process known as propagation losses.
The actual ultrasonic attenuation characteristics of a composite structure depends on the properties of the structure, the roughness of its surfaces and the materials at the front and back surfaces of the structure. A system which is commonly known as an ultrasonic testing (UT) flaw detector may be used to measure the ultrasonic attenuation characteristics of a composite structure. A UT flaw detector typically comprises at least a voltage pulser/receiver, signal display capability, signal cable, and ultrasonic transducer. Because each UT flaw detector may have its own effective frequency bandwidth which differs from that of other flaw detectors, two different UT flaw detectors may measure different attenuations on the same composite part, even if both flaw detectors are configured to run at the same frequency (such as 1 MHz). Because propagation attenuation due to porosity is a strong function of ultrasound frequency, different porosity readings for the same composite structure may be obtained using different UT flaw detectors. Therefore, physical porosity standards may be used to standardize the UT flaw detectors for measuring the porosity of composite structures.
One of the drawbacks of using physical porosity standards to standardize UT flaw detectors used in composite structure porosity measurement is that the physical porosity standards are expensive since they are difficult and time-consuming to produce and verify. Moreover, due to the methods by which they are fabricated, physical porosity standards may suffer from some variation in porosity from one batch to another. Shipment of these physical porosity standards around the world to sites where they are used may be necessary. Consequently, the physical porosity standards may not be available when needed. Production of a new set of porosity standards may be an expensive, difficult and time-consuming undertaking.
Therefore, a method and devices to standardize flaw detection using ultrasonic flaw detectors in which electronic porosity standards are used instead of physical porosity standards are needed.